fix duplicate installation in build.sh

[popfido]

• previously build.sh will try to install xlearn to all versions of python in a machine, which is quite unreasonable. This commit fix it and now build.sh only install xlearn to the current command line env's default python

[pkwv2012]

how about using the interactive install?
@popfido @aksnzhy

install xlearn for Python2? [y/n]
y
install xlearn for Python3? [y/n]
n

[popfido]

@purkyston I think ppl have clear acknowledgement that when they run this shell script, they want xlearn to be installed "exactly" into the python which the shell env currently use by default. If they want to install it into a non-default python, they will firstly enable the corresponding virtual env to ensure the "python" in current commandline env is the one they want to install xlearn.

@aksnzhy How do u think?