Travis config update: compile against minimum supported v8 and most recent v8.

.travis.yml

@@ -3,6 +3,9 @@ php:
   - 5.5
   - 5.4
   - 5.3
+env:
+  - V8VER=3.22.22
+  - V8VER=3.21.12
 before_install: make -f Makefile.travis before_install
 install: make -f Makefile.travis install
 script: make -f Makefile.travis test

Makefile.travis

@@ -1,6 +1,6 @@
 # Configure and build scripts for travis CI system
-V8VER=3.22.10
-CPPVER=4.6
+V8VER ?= 3.22.10
+CPPVER ?= 4.6
 
 export CXX=g++-$(CPPVER)
 export LINK=g++-$(CPPVER)