Install matplotlib on travis with max version 3.0.0 to avoid basemap bug

See matplotlib/basemap#435

Author: jmozmoz

.travis.yml

@@ -36,6 +36,8 @@ script:
   - pip install cython
   - pip install pyproj
   - pip install ${PKG}[debug_cartopy]
+  - pip uninstall matplotlib -y
+  - pip install "matplotlib<=3.0.0" 
   - pip install git+https://github.com/matplotlib/basemap
   - pip install ${PKG}[debug_pyresample]
   - create_map -d -m 23