Add patch to allow build with clang 7.3

.travis.yml

@@ -11,3 +11,4 @@ notifications:
 before_install:
   - sudo apt-get update
   - sudo apt-get install git-svn
+  - gem update bundler

ext/libv8/patcher.rb

@@ -20,6 +20,7 @@ def patch_directories_for(compiler)
         patch_directories << 'clang33' if compiler.version >= '3.3'
         patch_directories << 'clang51' if compiler.version >= '5.1'
         patch_directories << 'clang70' if compiler.version >= '7.0'
+        patch_directories << 'clang73' if compiler.version >= '7.3'
       end
 
       patch_directories

patches/clang73/no-shift-negative-value.patch

@@ -0,0 +1,10 @@
+--- a/build/standalone.gypi
++++ b/build/standalone.gypi
+@@ -215,6 +215,7 @@
+             '-Wno-unused-variable',
+             '-Wno-unused-local-typedefs',
+             '-Wno-tautological-undefined-compare',
++            '-Wno-shift-negative-value',
+             '-Wnon-virtual-dtor',
+           ],
+         },