Merge branch 'ls/travis-prove-order'

Automated tests in Travis CI environment has been optimized by
persisting runtime statistics of previous "prove" run, executing
tests that take longer before other ones; this reduces the total
wallclock time.

* ls/travis-prove-order:
  travis-ci: explicity use container-based infrastructure
  travis-ci: run previously failed tests first, then slowest to fastest

.travis.yml

@@ -1,5 +1,11 @@
 language: c
 
+sudo: false
+
+cache:
+  directories:
+    - $HOME/travis-cache
+
 os:
   - linux
   - osx
@@ -18,7 +24,7 @@ env:
     - P4_VERSION="15.2"
     - GIT_LFS_VERSION="1.1.0"
     - DEFAULT_TEST_TARGET=prove
-    - GIT_PROVE_OPTS="--timer --jobs 3"
+    - GIT_PROVE_OPTS="--timer --jobs 3 --state=failed,slow,save"
     - GIT_TEST_OPTS="--verbose --tee"
     - CFLAGS="-g -O2 -Wall -Werror"
     - GIT_TEST_CLONE_2GB=YesPlease
@@ -67,6 +73,8 @@ before_install:
     p4 -V | grep Rev.;
     echo "$(tput setaf 6)Git-LFS Version$(tput sgr0)";
     git-lfs version;
+    mkdir -p $HOME/travis-cache;
+    ln -s $HOME/travis-cache/.prove t/.prove;
 
 before_script: make --jobs=2