Fix tests to pass with the latest Nim

There are two issues fixed:

 - With the latest Nim version sometimes the files
   `<package_name>_<some_number>.nims`' generated on `nim install`
   contain warning for unused imports which causes the test
   "can validate package structure (#144)" to fail, because it was
   searching for the word "warning" in the output.

 - On Windows Subsystem for Linux, when an import starts sometimes with
   a lowercase, and sometimes with an uppercase, for example
   `import uri` and `import Uri`, this causes Nim to create and compile
   both `stdlib_uri.nim.c` and `stdlib_Uri.nim.c` and to fail on the
   linking step, because of the same symbols are being redefined.

Also the Travis CI build script is changed to test against currently the
latest working Nim version 212ae2f.

Related to #680

.travis.yml

@@ -7,7 +7,8 @@ language: c
 env:
   - BRANCH=0.19.6
   - BRANCH=0.20.2
-  - BRANCH=#44aadd50cfa647a759610a15967960632bf597ce
+  # This is the latest working Nim version against which Nimble is being tested
+  - BRANCH=#212ae2f1257628bd5d1760593ce0a1bad768831a
 
 cache:
   directories: