MNT: Remove 3.3 builds.

Matplotlib >= 1.5 only supports Python >= 3.4, so given we've dropped
matplotlib 1.4, it doesn't make sense to support 3.3.

Author: dopplershift

.travis.yml

@@ -13,7 +13,6 @@ env:
     - CONDA_DEPENDENCIES="pytest matplotlib nose coverage"
     - PIP_DEPENDENCIES="pytest-cov coveralls"
   matrix:
-    - PYTHON_VERSION=3.3 MATPLOTLIB_VERSION=1.5
     - PYTHON_VERSION=3.4 MATPLOTLIB_VERSION=1.5
     - PYTHON_VERSION=2.7 MATPLOTLIB_VERSION=1.5
     - PYTHON_VERSION=3.5 MATPLOTLIB_VERSION=1.5