Increase ccache size for debug builds

Currently we use a 1GB limit on the ccache size for all builds.
However for the debug builds (those specifying '-g' to g++) object
sizes are significantly larger due to the extra DWARF debug info.
This is causing the debug builds to hit the ccache size limit.
Local experiments suggest that with a 1GB limit, we can only get
about 80% hit rate. Increasing the ccache size to 2GB increases
the hit rate to 99%.

Author: chrisr-diffblue

.travis.yml

@@ -249,7 +249,7 @@ jobs:
 
 install:
   - ccache -z
-  - ccache --max-size=1G
+  - ccache --max-size=2G
   - make -C src minisat2-download
   - make -C src/ansi-c library_check
   - make -C src "CXX=${COMPILER}" "CXXFLAGS=-Wall -Werror -pedantic -O2 -g ${EXTRA_CXXFLAGS}" -j2