.travis.yml: work around caching and test configuration

Test configurations varying by env variables currently share the same
cache. This is a known issue
(travis-ci/travis-ci#4393) with a workaround
used by the bitcoin project
(https://github.com/bitcoin/bitcoin/blob/b5cbd396ca7214f4f944163ed314456038fdd818/.travis.yml).

We use the same workaround here.

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>

.travis.yml

@@ -1,4 +1,20 @@
 language: c++
+compiler: gcc
+# Workaround for caching issue (https://github.com/travis-ci/travis-ci/issues/4393):
+# cache selection depends on compiler, but not env variables. In our test matrix
+# we must use a fake compiler to get different caches.
+matrix:
+  include:
+    - compiler: ": master"
+      env: OE_CORE=master BITBAKE=master
+    - compiler: ": jethro"
+      env: OE_CORE=jethro BITBAKE=1.28
+    - compiler: ": fido"
+      env: OE_CORE=fido BITBAKE=1.26
+    - compiler: ": dizzy"
+      env: OE_CORE=dizzy BITBAKE=1.24
+  exclude:
+    - compiler: gcc
 addons:
   apt:
     packages:
@@ -7,12 +23,6 @@ addons:
     - chrpath
     # Not needed, sdl removed from qemu below.
     # - libsdl1.2-dev
-env:
-  # https://github.com/travis-ci/travis-ci/issues/5127
-  - OE_CORE=master BITBAKE=master
-  # - OE_CORE=jethro BITBAKE=1.28
-  # - OE_CORE=fido BITBAKE=1.26
-  # - OE_CORE=dizzy BITBAKE=1.24
 install:
   - git clone --depth=1 --single-branch --branch=$OE_CORE git://git.openembedded.org/openembedded-core
   - ( cd openembedded-core && git clone --depth=1 --single-branch --branch=$BITBAKE git://git.openembedded.org/bitbake )