Pin rake to 10.1.x because of Ruby 1.8

Rake 10.2 introduced a breaking requirement for Ruby 1.9 so our builds no
longer work on Ruby 1.8 -- pin the major/minor version for the moment until
we find a better solution.
```
Gem::InstallError: rake requires Ruby version >= 1.9.
An error occurred while installing rake (10.2.1), and Bundler cannot continue.
Make sure that `gem install rake -v '10.2.1'` succeeds before bundling.
```