travis CI changes

OSX build are taking too long to finish, mark these builds as optional (allow_failure), and then
set the 'fast_finish' flag so that we report the build as passing as soon as the linux builds
finish.

Also test out python3.6 on linux too, and build extras on linux as well.

.travis.yml

@@ -1,10 +1,17 @@
 language: cpp
+
 matrix:
+  fast_finish: true
   include:
   - os: linux
     env: PYTHON=2.7
+  - os: linux
+    env: PYTHON=3
   - os: osx
     env: PYTHON=3.6
+  allow_failures:
+  - os: osx
+
 addons:
   apt:
     sources:
@@ -15,6 +22,10 @@ addons:
     - liblapack-dev
     - gfortran
     - cmake
+    - python3-dev
+    - libboost-all-dev
+    - libgsl0-dev
+    - libeigen3-dev
 
 before_install:
 - |
@@ -30,9 +41,15 @@ before_install:
   source venv/bin/activate
 
 install:
-  - cmake similarity_search && make -j 4
-  - travis_wait travis_retry pip install -r python_bindings/requirements.txt scipy six flake8
-  - travis_retry cd python_bindings && python setup.py build install && cd ..
+- |
+  if [ "$TRAVIS_OS_NAME" = "linux" ]; then
+    cmake similarity_search -DWITH_EXTRAS=1
+  else
+    cmake similarity_search
+  fi
+  make -j 4
+  travis_wait travis_retry pip install -r python_bindings/requirements.txt scipy six flake8
+  travis_retry cd python_bindings && python setup.py build install && cd ..
 
 script:
 - |
@@ -46,6 +63,7 @@ script:
   python setup.py test
   flake8
   cd ..
+
 cache:
   - apt
   - directories: