bundle install failed ..

# Report

## What did you do?

ℹ Please replace this with what you did.
I follow the installation
`git clone https://github.com/danger/danger.git`
`cd danger`
`bundle install`

## What did you expect to happen?

I was expected a full installation.

## What happened instead?

[!] There was an error parsing Gemfile: There was a RuntimeError while loading danger.gemspec:
can t modify frozen String from /Users/me/danger/danger.gemspec:8:in block in
. Bundler cannot continue.

from /Users/me/danger/Gemfile:3
> gemspec
gem "danger-gitlab"

## Your Environment

* Running on macOSX Bundler version 1.10.6
* Bundler version 1.10.6
* ruby 2.0.0p648 (2015-12-16 revision 53162) [universal.x86_64-darwin16]

Inspect danger.gemspec at line 8 and reproduce the reported failure with the Ruby 2.0.0 and Bundler 1.10.6 environment described. Check the Gemfile entry that loads the gemspec, then verify that bundle install completes without the Gemfile or gemspec parsing error.

Written by the indexing model from the issue text.