bowtie-build changes

[MikeAxtell]

• Consider sending the output of bowtie-build to /dev/null instead of letting it spill onto the screen. It is verbose but means nothing for the user.
• bowtie has the following message: Setting the index via positional argument will be deprecated in a future release. Please use -x option instead.. You should change the bowtie command accordingly to ensure future compatibility with newer versions of bowtie.

[Aez35]

Included -x option and removed output from bowtie