Fix warnings in test suite and gemspec

The following warnings are fixed:

```
bundler.gemspec:6: warning: shadowing outer local variable - spec
/home/travis/build/bundler/bundler/spec/bundler/dsl_spec.rb:150: warning: assigned but unused variable - example
/home/travis/build/bundler/bundler/spec/bundler/dsl_spec.rb:165: warning: assigned but unused variable - example
```