Travis CI: fix old openssl in macos 10.12 and 10.11 (only tlsv1 available)

[maelvls]

Try to fix the error:

sudo gem install aws-sdk-s3

ERROR:  Could not find a valid gem 'aws-sdk-s3' (>= 0), here is why:
Unable to download data from https://rubygems.org/ - SSL_connect returned=1
errno=0 state=SSLv2/v3 read server hello A: tlsv1 alert protocol version
(https://rubygems.org/latest_specs.4.8.gz)
The command "sudo gem install aws-sdk-s3" failed and exited with 2.

[maelvls]

I realized this is because I use the system's curl Why? Because I uninstall brew for testing it, which removes curl installed by brew

[maelvls]

Fixed in #13