Add python2.7-dev to the commands for installing dependencies.
Add python2.7-dev to the commands for installing dependencies.

 * Change commands to explicitly require Python2.7, because we can guarantee
   that version works.

DoNotUseThisCodeJUSTFORKS/ooni-backend,DoNotUseThisCodeJUSTFORKS/ooni-backend,dstufft/ooni-backend,dstufft/ooni-backend