added rest-client to gemspec as dependency
added rest-client to gemspec as dependency

Signed-off-by: Akash Manohar J <e4cf979cf717fd295f4e4edc427ee1aee78fa7dc@akash.im>

KevinColemanInc/mailgun,HashNuke/mailgun,bseth99/mailgun,KevinColemanInc/mailgun,HashNuke/mailgun