[travis] install a specific version of mirage
[travis] remove constraint on mirage.0.9.7 and see if it works now
blkfront: the request should include the unaligned sector

This fixes non-page aligned reads.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #4 from djs55/fix-read-sectors

blkfront: the request should include the unaligned sector