Add check for breaking API changes

This should be able to detect breaking API changes in pull
requests. When the breaking changes are by design we must ignore
the failing CI.

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>

.travis.yml

@@ -8,12 +8,15 @@ before_install:
   - go get github.com/mattn/goveralls
   - go get golang.org/x/tools/cmd/cover
   - go get -u github.com/golang/lint/golint
+  - go get -u github.com/bradleyfalzon/apicompat/cmd/apicompat
 
 script:
   - go vet -x ./...
   - golint ./...
   - go test -v  ./...
   - go test -covermode=count -coverprofile=profile.cov
+  - git --git-dir /home/travis/gopath/src/github.com/ahmetalpbalkan/go-linq/.git ls-tree master ./ || true
+  - test -z "$(apicompat -before ${TRAVIS_COMMIT_RANGE%...*} -after ${TRAVIS_COMMIT_RANGE#*...} ./... | tee /dev/stderr)"
 
 after_script:
-  - goveralls -coverprofile=profile.cov -service=travis-ci
+  - goveralls -coverprofile=profile.cov -service=travis-ci