Cleanup up minify js script

- The slimit package is now widely available so make the script
   only rely on it and closure. Keeping closure as it makes the most
   compact and lints the javascript but it is not as easy to install
   as slimit.

minify_web_js.py

@@ -20,6 +20,7 @@ from __future__ import print_function
 import fileinput
 import fnmatch
 import os
+import subprocess
 import sys
 
 
@@ -35,21 +36,16 @@ def module_exists(module_name):
 # Imports sorted by resulting file size.
 if module_exists('closure'):
     def minify_closure(file_in, file_out):
-        import subprocess
         try:
             subprocess.check_call(['closure', '-W', 'QUIET',
                                    '--js', file_in, '--js_output_file', file_out])
             return True
         except subprocess.CalledProcessError:
             return False
-elif module_exists('slimit'):
+elif module_exists('sslimit'):
     from slimit import minify
-elif module_exists('jsmin'):
-    from jsmin import jsmin as minify
-elif module_exists('rjsmin'):
-    from rjsmin import jsmin as minify
 else:
-    raise ImportError('Minifying WebUI JS requires slimit, jsmin or rjsmin')
+    raise ImportError('Requires "slimit" package for minifying WebUI files.')
 
 
 def source_files_list(source_dir):
@@ -105,9 +101,8 @@ def minify_js_dir(source_dir):
         return
 
     concat_src_files(source_files, file_debug_js)
-    if minify_file(file_debug_js, file_minified_js):
-        print('Minified %s' % source_dir)
-    else:
+    print('Minifying %s' % source_dir)
+    if not minify_file(file_debug_js, file_minified_js):
         print('Error minifying %s' % source_dir)