Attempt to run tests on multiple Firefox versions (#39)

.travis.yml

@@ -5,18 +5,58 @@ python:
 
 sudo: "required"
 
-addons:
-  firefox: "latest-nightly"
-
 env:
   global:
     - DISPLAY=:99.0
 matrix:
   include:
     - python: 2.7
       env: TOXENV=flake8
+    - python: 2.7
+      env: FIREFOX=nightly
+      addons:
+        firefox: latest-nightly
+    - python: 2.7
+      env: FIREFOX=dev
+      addons:
+        firefox: latest-dev
+    - python: 2.7
+      env: FIREFOX=beta
+      addons:
+        firefox: latest-beta
+    - python: 2.7
+      env: FIREFOX=release
+      addons:
+        firefox: latest
+    - python: 2.7
+      env: FIREFOX=esr
+      addons:
+        firefox: latest-esr
     - python: 3.5
       env: TOXENV=flake8
+    - python: 3.5
+      env: FIREFOX=nightly
+      addons:
+        firefox: latest-nightly
+    - python: 3.5
+      env: FIREFOX=dev
+      addons:
+        firefox: latest-dev
+    - python: 3.5
+      env: FIREFOX=beta
+      addons:
+        firefox: latest-beta
+    - python: 3.5
+      env: FIREFOX=release
+      addons:
+        firefox: latest
+    - python: 3.5
+      env: FIREFOX=esr
+      addons:
+        firefox: latest-esr
+  exclude:
+    - python: 2.7
+    - python: 3.5
 
 before_install:
   - wget -O geckodriver.tar.gz https://github.com/mozilla/geckodriver/releases/download/v0.11.1/geckodriver-v0.11.1-linux64.tar.gz
@@ -38,4 +78,4 @@ before_script:
   - sh -e /etc/init.d/xvfb start
 
 script:
-  - tox
+  - tox