We cannot just 'git checkout CIRCLE_BRANCH_NAME' because
for pull requests CIRCLE_BRANCH_NAME is just pull/ID and there
is no such branch. Need to use 'git fetch $CIRCLE_BRANCH_NAME/head'
which then get pulled into local branch called FETCH_HEAD, where we
can do build and rspec
We cannot just 'git checkout CIRCLE_BRANCH_NAME' because for pull requests CIRCLE_BRANCH_NAME is just pull/ID and there is no such branch. Need to use 'git fetch $CIRCLE_BRANCH_NAME/head' which then get pulled into local branch called FETCH_HEAD, where we can do build and rspec