gemspec: use Sinatra 1.2.0 or higher
gemspec: use Sinatra 1.2.0 or higher

SequenceServer uses Sinatra's `uri` helper which is available only in Sinatra
1.2.0 and higher.

Signed-off-by: Anurag Priyam <6e6ab5ea9cb59fe7c35d2a1fc74443577eb60635@gmail.com>

deepakunni3/sequenceserver,lepbase/lepbase-sequenceserver,jasminZ/sequenceserver,aldermossy/sequenceserver,IsmailM/sequenceserver,vivekiitkgp/sequenceserver,elsiklab/sequenceserver,elsiklab/sequenceserver,deepakunni3/sequenceserver,jasminZ/sequenceserver,yeban/sequenceserver,vivekiitkgp/sequenceserver,aldermossy/sequenceserver,yeban/sequenceserver,elsiklab/sequenceserver,jasminZ/sequenceserver,aldermossy/sequenceserver,IsmailM/sequenceserver,yeban/sequenceserver,lepbase/lepbase-sequenceserver,deepakunni3/sequenceserver,IsmailM/sequenceserver,yeban/sequenceserver,lepbase/lepbase-sequenceserver,aldermossy/sequenceserver,vivekiitkgp/sequenceserver