Disable Romio tests in Travis build

[sbesson]

Lately Romio tests have been hanging frequently on Travis causing the build to fail.

[joshmoore]

With this green, merging as an emergency effort. If it doesn't fix the issue, we can rollback.

[joshmoore]

Note: #3880 which seems to have been closed/re-opened with this in failed due to:

[nomemorytestng] FAILED: testBasicUsage on null(omero.util.test.TempFileManagerTest)
[nomemorytestng] org.testng.internal.thread.ThreadTimeoutException: Method org.testng.internal.TestNGMethod.testBasicUsage() didn't finish within the time-out 1000
[nomemorytestng]    at java.io.UnixFileSystem.createFileExclusively(Native Method)
[nomemorytestng]    at java.io.File.createNewFile(File.java:1006)
[nomemorytestng]    at java.io.File.createTempFile(File.java:1989)
[nomemorytestng]    at omero.util.TempFileManager.tmpdir(TempFileManager.java:225)
[nomemorytestng]    at omero.util.TempFileManager.<init>(TempFileManager.java:103)
[nomemorytestng]    at omero.util.TempFileManager.<init>(TempFileManager.java:93)
[nomemorytestng]    at omero.util.TempFileManager.<clinit>(TempFileManager.java:60)

so perhaps whatever was causing the romio issue also causes this hang??