Copy repository to $GOPATH and build from $GOPATH

@joshk The specs fails since there's a spec that checks `$TRAVIS_BUILD_DIR` for every language to be `~/builds/owner/repo`, although we technically don't guarantee for it to be that. Any suggestions on how to fix this?

@titanous @peterbourgon @sam-falvo Does this look like a good change? The build script will end up looking something like this:

```
cd ~/build
export TRAVIS_BUILD_DIR=~/build/travis-ci/travis-ci
export GOPATH=~/gopath
git clone ... travis-ci/travis-ci
mkdir $GOPATH/src/github.com/travis-ci/travis-ci
cp -r $TRAVIS_BUILD_DIR/ $GOPATH/src/github.com/travis-ci/travis-ci
export TRAVIS_BUILD_DIR=$GOPATH/src/github.com/travis-ci/travis-ci
cd $GOPATH
```