BLD: Bump pandas version to 0.15 or newer

Some of the functionality used in
`qiita_db.metadata_template.load_template_to_dataframe` is only
available in 0.15 and newer.

Fixes #675
DOC: Update INSTALL.md
Merge pull request #683 from ElDeveloper/issue-675

BLD: Bump pandas version to 0.15 or newer