Add the toolchain api test to the travis CI

I would like to test that the Toolchains API remains stable. Therfore,
I have added a test to the travis CI that checks that each exposed
toolchain class implements the API that the mbedToolchain base class
uses to compile.

.travis.yml

@@ -3,6 +3,7 @@ python:
 
 script:
   - PYTHONPATH=. python tools/test/config_test/config_test.py
+  - py.test tools/test/toolchains/api.py
   - python tools/build_travis.py
 before_install:
   - sudo add-apt-repository -y ppa:terry.guo/gcc-arm-embedded
@@ -15,3 +16,4 @@ install:
   - sudo pip install colorama
   - sudo pip install prettytable
   - sudo pip install jinja2
+  - sudo pip install pytest