Build on travis with CPUS=2

With CPU=3 we are hitting resoure limits (probably memory). With CPU=2
this is less likely to happen. We will get more random timeouts, but
these are more easily spotted, and marked as "Failure", not as "Error".

.travis.yml

@@ -33,4 +33,4 @@ script:
  - echo 'DYNAMIC_GHC_PROGRAMS = NO' >> mk/validate.mk
  - echo 'GhcLibWays = v' >> mk/validate.mk
  - if [ "$DEBUG_STAGE" = "YES" ]; then echo 'GhcStage2HcOpts += -DDEBUG' >> mk/validate.mk; fi
- - CPUS=3 SKIP_PERF_TESTS=YES PATH=~/.cabal/bin:$PATH ./validate --fast --no-dph
+ - CPUS=2 SKIP_PERF_TESTS=YES PATH=~/.cabal/bin:$PATH ./validate --fast --no-dph