Let's say you want to use a gem in a plugin that has a dependencie.
You would write something like this:
```ruby
gem 'dependency-gem', '1.2.3'
gem 'amazing-gem', '4.5.6'
```
However, since when we install a plugin gem we install it in the `gems`
directory created inside the plugins directory, when it comes the time
to install the `amazing-gem`, it won't be able to find the `dependency-gem`.
This fixes that issue by adding the `gems` plugins directory to the global gem path.
Also fixed a frozen string error when specifying a source.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
Régis Hanol
Faizaan Gagan
Sam Saffron
Matt Palmer
Robin Ward
Guo Xiang Tan