travis: add coveralls.io support

I suggest to add support for coveralls.io.
It can't show all coverage, as we don't run all tests on travis,
but still everyone who forked CRIU can see instant coverage of
his Travis build. As a nice feature, it will show code cover
increasing/decreasing just after you push a commit - which
could give you a nice hint about content of a new tests.
For this feature, we need to build CRIU with `make GCOV=1`,
which adds --coverage and disables optimizations.

The resulting report looks like this:
https://coveralls.io/builds/6439346
I was thinking about codecov.io also, but it has fewer community,
less fixed issues on github - so I choose this service.

Just a random fun fact:
file-ids.c covered completely on tests, except -ENOMEM return.
Maybe it worth to run fault injections tests on Travis.
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

.travis.yml

@@ -11,3 +11,8 @@ env:
 script:
   - sudo make -C scripts/build $TR_ARCH
   - sudo ./scripts/travis-tests $TR_ARCH
+after_success:
+  - sudo apt-get install -qq -y lcov
+  - gem install coveralls-lcov
+  - sudo lcov --directory criu --capture --output-file coverage.info
+  - coveralls-lcov coverage.info

scripts/travis-tests

@@ -6,7 +6,7 @@ test "$1" = "x86_64" || exit 0
 apt-get update -qq
 apt-get install -qq protobuf-c-compiler libprotobuf-c0-dev libaio-dev libprotobuf-dev protobuf-compiler python-ipaddr libcap-dev libnl-3-dev
 chmod a+x $HOME
-make
+make GCOV=1
 make -C test/zdtm
 python test/zdtm.py run -a -f h,ns -x 'cgroup*'