add CI to set the stage for running fontbakery (#38)

- fontmake cannot be run yet, it errors (https://github.com/chrismsimpson/Metropolis/pull/35/commits/1b143bec257e5c2d37d760f4eb83db4219e88c75 seems to indicate that the following will fix the build issues: adding a Light[700] bracket layer for each of the glyphs that were causing fontmake builds to fail: the cent and the dollar on both the Regular and the Italic)
- subsequent steps are commented out so that they can be enabled progressively in subsequent PRs

.travis.yml

@@ -0,0 +1,8 @@
+language: python
+
+install:
+  - pip install --upgrade pip
+  - pip install -r Sources/requirements.txt
+
+script:
+- ./Sources/build.sh