Timex fails to pick up timezone on TrueOS (and possibly FreeBSD too)

### Steps to reproduce

```elixir
iex> Timex.local()
```

### Description of issue

```elixir
iex(5)> Timex.local()
** (MatchError) no match of right hand side value: nil
(timex) lib/timezone/timezone_local.ex:123: Timex.Timezone.Local.localtz/2
(timex) lib/timezone/timezone_local.ex:58: Timex.Timezone.Local.lookup/1
(timex) lib/timezone/timezone.ex:53: Timex.Timezone.local/1
(timex) lib/timex.ex:64: Timex.local/0
```

### Workaround

```bash
$ sudo -s
# echo Europe/Budapest > /etc/timezone
```

### Environment

The proper timezone had been configured to TrueOS with `tzsetup`, but this doesn't generate an `/etc/timezone` file. This information is recorded in `/var/db/zoneinfo`. Erlang and Elixir had been compiled from source. I used `elixir-1.8.1` and `otp_src_21.2`.

```bash
$ cat /var/db/zoneinfo
Europe/Budapest
```

```bash
$ uname -a
FreeBSD some_server 13.0-CURRENT FreeBSD 13.0-CURRENT fbd2a3ef6(HEAD) GENERIC amd64
```

```bash
$ erl
Erlang/OTP 21 [erts-10.2] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1] [hipe]
Eshell V10.2 (abort with ^G)
```

```bash
$ iex
Erlang/OTP 21 [erts-10.2] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1] [hipe]
Interactive Elixir (1.8.1) - press Ctrl+C to exit (type h() ENTER for help)
```

```bash
$ mix deps.tree
...
├── timex ~> 3.4 (Hex package)
│ ├── combine ~> 0.10 (Hex package)
│ ├── gettext ~> 0.10 (Hex package)
│ └── tzdata ~> 0.1.8 or ~> 0.5 (Hex package)
│ └── hackney ~> 1.0 (Hex package)
│ ├── certifi 2.4.2 (Hex package)
│ │ └── parse_trans ~>3.3 (Hex package)
│ ├── idna 6.0.0 (Hex package)
│ │ └── unicode_util_compat 0.4.1 (Hex package)
│ ├── metrics 1.0.1 (Hex package)
│ ├── mimerl 1.0.2 (Hex package)
│ └── ssl_verify_fun 1.1.4 (Hex package)
```

```bash
$ cat mix.lock
%{
...
"certifi": {:hex, :certifi, "2.4.2", "75424ff0f3baaccfd34b1214184b6ef616d89e420b258bb0a5ea7d7bc628f7f0", [:rebar3], "combine": {:hex, :combine, "0.10.0", "eff8224eeb56498a2af13011d142c5e7997a80c8f5b97c499f84c841032e429f", [:mix], [], "hexpm"},
"gettext": {:hex, :gettext, "0.16.0", "4a7e90408cef5f1bf57c5a39e2db8c372a906031cc9b1466e963101cb927dafc", [:mix], [], "hexpm"},
"hackney": {:hex, :hackney, "1.14.3", "b5f6f5dcc4f1fba340762738759209e21914516df6be440d85772542d4a5e412", [:rebar3], [{:certifi, "2.4.2", [hex: :certifi, repo: "hexpm", optional: false]}, {:idna, "6.0.0", [hex: :idna, repo: "hexpm", optional: false]}, {:metrics, "1.0.1", [hex: :metrics, repo: "hexpm", optional: false]}, {:mimerl, "1.0.2", [hex: :mimerl, repo: "hexpm", optional: false]}, {:ssl_verify_fun, "1.1.4", [hex: :ssl_verify_fun, repo: "hexpm", optional: false]}], "hexpm"},
"idna": {:hex, :idna, "6.0.0", "689c46cbcdf3524c44d5f3dde8001f364cd7608a99556d8fbd8239a5798d4c10", [:rebar3], [{:unicode_util_compat, "0.4.1", [hex: :unicode_util_compat, repo: "hexpm", optional: false]}], "hexpm"},
"metrics": {:hex, :metrics, "1.0.1", "25f094dea2cda98213cecc3aeff09e940299d950904393b2a29d191c346a8486", [:rebar3], [], "hexpm"},
"mimerl": {:hex, :mimerl, "1.0.2", "993f9b0e084083405ed8252b99460c4f0563e41729ab42d9074fd5e52439be88", [:rebar3], [], "hexpm"},
"parse_trans": {:hex, :parse_trans, "3.3.0", "09765507a3c7590a784615cfd421d101aec25098d50b89d7aa1d66646bc571c1", [:rebar3], [], "hexpm"},
"ssl_verify_fun": {:hex, :ssl_verify_fun, "1.1.4", "f0eafff810d2041e93f915ef59899c923f4568f4585904d010387ed74988e77b", [:make, :mix, :rebar3], [], "hexpm"},
"timex": {:hex, :timex, "3.4.1", "e63fc1a37453035e534c3febfe9b6b9e18583ec7b37fd9c390efdef97397d70b", [:mix], [{:combine, "~> 0.10", [hex: :combine, repo: "hexpm", optional: false]}, {:gettext, "~> 0.10", [hex: :gettext, repo: "hexpm", optional: false]}, {:tzdata, "~> 0.1.8 or ~> 0.5", [hex: :tzdata, repo: "hexpm", optional: false]}], "hexpm"},
"tzdata": {:hex, :tzdata, "0.5.19", "7962a3997bf06303b7d1772988ede22260f3dae1bf897408ebdac2b4435f4e6a", [:mix], [{:hackney, "~> 1.0", [hex: :hackney, repo: "hexpm", optional: false]}], "hexpm"},
"unicode_util_compat": {:hex, :unicode_util_compat, "0.4.1", "d869e4c68901dd9531385bb0c8c40444ebf624e60b6962d95952775cac5e90cd", [:rebar3], [], "hexpm"},
}
```