permissions error on linux

## Prerequisites

* [ x] I am running the latest version. (`up upgrade`)
* [ x] I searched to see if the issue already exists.
* [ ] ~~I inspected the verbose debug output with the `-v, --verbose` flag.~~ N/A

## Description
Running install script, either via curl or npm, throws a permissions error. unsure if this is related to #337 or not. Other global installs (yarn, flow, etc) work fine.

```
$ npm i -g up

> up@1.0.1 postinstall /home/bmp/.nvm/versions/node/v8.1.1/lib/node_modules/up
> curl -sfL https://raw.githubusercontent.com/apex/up/master/install.sh | sh

apex/up: checking GitHub for latest version
apex/up: found version 0.2.6 for linux/amd64
apex/up: downloading https://github.com/apex/up/releases/download/v0.2.6/up_0.2.6_linux_amd64.tar.gz
apex/up: verifying checksums
install: cannot create regular file '/usr/local/bin/up': Permission denied
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! up@1.0.1 postinstall: `curl -sfL https://raw.githubusercontent.com/apex/up/master/install.sh | sh`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the up@1.0.1 postinstall script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR! /home/bmp/.npm/_logs/2017-10-07T14_22_41_144Z-debug.log

```

## Steps to Reproduce

run install as per docs on a system like mine:
Ubuntu Mate 17.04
node 8.1.1 / npm 5.0.3

## Love Up?

I want to! But I'm not the type of guy to casually toss-around `sudo Love`, you know?