server certificate verification failed

## Describe the problem
The installer fails to install do-agent.

## Steps to reproduce
```
$ curl -sSL https://repos.insights.digitalocean.com/install.sh -o /tmp/install.sh
$ sudo bash /tmp/install.sh
Cleaning up old sources...OK
Verifying machine compatibility...OK
Verifying compatibility with script...OK
Installing apt repository...
Installing gpg key...OK
W: Failed to fetch https://repos.insights.digitalocean.com/apt/do-agent/dists/main/main/binary-amd64/Packages server certificate verification failed. CAfile: /etc/ssl/certs/ca-certificates.crt CRLfile: none

W: Failed to fetch https://repos.insights.digitalocean.com/apt/do-agent/dists/main/main/binary-i386/Packages gnutls_handshake() failed: Decrypt error

E: Some index files failed to download. They have been ignored, or old ones used instead.
```

## Expected behavior
We expect do-agent to be properly installed.

## System Information
Distribution and version:
```
$ cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=14.04
DISTRIB_CODENAME=trusty
DISTRIB_DESCRIPTION="Ubuntu 14.04.2 LTS"
```