Skip File.atime/File.mtime tests randomly failing on Travis

Not only powerpc64le, also s390x and arm32 seem failing too.  These
failures are probably caused by filesystem settings on Travis, but
unrelated to CPUs.

spec/ruby/core/file/atime_spec.rb

@@ -16,7 +16,7 @@ describe "File.atime" do
   end
 
   platform_is :linux, :windows do
-    platform_is_not :"powerpc64le-linux" do # https://bugs.ruby-lang.org/issues/17926
+    unless ENV.key?('TRAVIS') # https://bugs.ruby-lang.org/issues/17926
       ## NOTE also that some Linux systems disable atime (e.g. via mount params) for better filesystem speed.
       it "returns the last access time for the named file with microseconds" do
         supports_subseconds = Integer(`stat -c%x '#{__FILE__}'`[/\.(\d{1,6})/, 1], 10)

spec/ruby/core/file/mtime_spec.rb

@@ -16,6 +16,7 @@ describe "File.mtime" do
   end
 
   platform_is :linux, :windows do
+    unless ENV.key?('TRAVIS') # https://bugs.ruby-lang.org/issues/17926
       it "returns the modification Time of the file with microseconds" do
         supports_subseconds = Integer(`stat -c%y '#{__FILE__}'`[/\.(\d{1,6})/, 1], 10)
         if supports_subseconds != 0
@@ -27,6 +28,7 @@ describe "File.mtime" do
         end
       end
     end
+  end
 
   it "raises an Errno::ENOENT exception if the file is not found" do
     -> { File.mtime('bogus') }.should raise_error(Errno::ENOENT)