removed commented line in .travis.yaml

(cherry picked from commit 8b901c6a016f39cba0d851de5b08116d756b4bcd)
removed None path tests
jpype backend changed to raise ValueError on an empty path
removed libboost dependency
minor changes in .travis
added HDFS_BACKEND env in travis; removed temporarily all hadoop version except 2.2.0
Formatting
More work on hdfs_file
jpype backend: fixed hdfs file write
added jpype installation
added --allow-external arg for pip
fixed pip args
added --allow-unverified for pip jpype
downloading jpype from sourceforge instead of pip...
using apt for installing jpype
jpype backend: {p,}read_chunk changed to support bytearrays
fixed url for jpype download
restore working dir after installing jpype
using pip install JPype1
fixed jpype installation