Lock Thin to 1.6.x

Tested-by: Mark Somerville <marks@1partcarbon.co.uk> [RHEL7, MRI 2.1.3]
Signed-off-by: Mark Somerville <mark@crocodile.is>
README: don't install gems in the test group

Signed-off-by: Mark Somerville <mark@crocodile.is>