specified pandas<0.21 in travis.yml file to avoid crash of Travis with version 0.21 of pandas.

Author: alixdamman

.travis.yml

@@ -46,7 +46,7 @@ install:
     # might want to avoid the later by installing all dependencies manually
     # except scipy and install pandas with --no-deps
     - conda create -n travisci --yes python=${TRAVIS_PYTHON_VERSION:0:3}
-            numpy pandas pytables pyqt qtpy matplotlib xlrd openpyxl
+            numpy "pandas<0.21" pytables pyqt qtpy matplotlib xlrd openpyxl
             xlsxwriter pytest pytest-qt
     - source activate travisci