For Travis CI - For MinGW STATIC build, retry sample build for 3 times.

Each retry should build starts from where it fails, so hopefully it would not hit the memory limit.
Closes #431.

Rakefile

@@ -275,9 +275,9 @@ def makefile_travis_ci
   else
     platform_prefix = ''
   end
-  # Temporary workaround due to Travis-CI insufficient memory in 64-bit/MinGW/STATIC build configuration, build samples separately using single worker process
-  if ENV['CI'] and ENV['WINDOWS'] and ENV['URHO3D_64BIT'] and ENV['URHO3D_LIB_TYPE'] == 'STATIC'
-    system "cd #{platform_prefix}Build/Tools && make -j$NUMJOBS && cd .. && make" or abort 'Failed to build or test Urho3D library'
+  # Temporary workaround due to Travis-CI insufficient memory in MinGW/STATIC build configuration, build samples separately and retry build for 3 times from where it fails
+  if ENV['CI'] and ENV['WINDOWS'] and ENV['URHO3D_LIB_TYPE'] == 'STATIC'
+    system "cd #{platform_prefix}Build/Tools && make -j$NUMJOBS && cd .. && (make -j$NUMJOBS || make -j$NUMJOBS || make -j$NUMJOBS)" or abort 'Failed to build or test Urho3D library'
   else
     # Only 64-bit Linux environment with virtual framebuffer X server support and not MinGW build; or OSX build environment are capable to run tests
     if $testing == 1 and (ENV['URHO3D_64BIT'] and ENV['WINDOWS'].to_i != 1 or ENV['OSX'])