travis: enable travis-ci integration

The script currently checks out frenetic-lang dependencies from github
and builds and installs them from source.
travis: add build status badge to README
Fix OF 1.0 per Marco's design
Fix conformance to specifications for Set-Field VLAN_VID
OpenFlow Specification 1.3.3 (https://www.opennetworking.org/images/stories/downloads/sdn-resources/onf-specifications/openflow/openflow-spec-v1.3.3.pdf), p. 65:
"The value in the payload of the OXM TLV must be valid, in particular the OFPVID_PRESENT bit must be set in OXM_OF_VLAN_VID set-field actions."
Just a couple of changes to maintain the original logical flow, reenable error handling and simplify the code.
Merge pull request #34 from frenetic-lang/handshake-fix

Fix OF 1.0 per Marco's design
Merge pull request #37 from frenetic-lang/fix-vlanid-present

Fix conformance to specifications for Set-Field VLAN_VID
Merge pull request #33 from frenetic-lang/travis

Travis-CI integration
travis: fix bug in build/test script

The script was previously using `git show --pretty=format:%H` to print
out the commit hash of dependencies checked out of github. But also,
this command will still prints out the commit diff. For merge commits,
and non-merge commits whose diff isn't long enough for require the use
of a pager, this is not a problem. However, if the commit diff is long
enough that git pipes it to a pager, it will cause the build to hang.
Instead of using `git show`, the script now uses `git rev-parse HEAD`,
which will only print out the commit hash of HEAD.
Merge pull request #41 from frenetic-lang/travis

fix bug in build/test script
Merge branch 'master' into enqueue