Made SyntacticIsVersionAtLeast more robust

It now handles more cases, which will cause Syntastic to degrade better
when commands return weird versions (e.g. the command crashes).

plugin/syntastic.vim

@@ -509,17 +509,34 @@ function! s:uname()
     return s:uname
 endfunction
 
-"the args must be arrays of the form [major, minor, macro]
+" Verify that the 'installed' version is at the 'required' version, if not
+" better.
+"
+" 'installed' and 'required' must be arrays.  Only the
+" first three elements (major, minor, patch) are looked at.
+"
+" Either array may be less than three elements. The "missing" elements
+" will be assumed to be '0' for the purposes of checking.
+"
+" See http://semver.org for info about version numbers.
 function SyntasticIsVersionAtLeast(installed, required)
-    if a:installed[0] != a:required[0]
-        return a:installed[0] > a:required[0]
-    endif
-
-    if a:installed[1] != a:required[1]
-        return a:installed[1] > a:required[1]
-    endif
-
-    return a:installed[2] >= a:required[2]
+    for index in [0,1,2]
+        if len(a:installed) <= index
+            let installed_element = 0
+        else
+            let installed_element = a:installed[index]
+        endif
+        if len(a:required) <= index
+            let required_element = 0
+        else
+            let required_element = a:required[index]
+        endif
+        if installed_element != required_element
+            return installed_element > required_element
+        endif
+    endfor
+    " Everything matched, so it is at least the required version.
+    return 1
 endfunction
 
 "return a string representing the state of buffer according to