only write file in jsversion if there's a change to make

avoids updating the file's mtime unnecessarily, forcing a rebuild

setupbase.py

@@ -470,18 +470,28 @@ class JavascriptVersion(Command):
     
     def run(self):
         nsfile = pjoin(repo_root, "notebook", "static", "base", "js", "namespace.js")
-        with open(nsfile) as f:
-            lines = f.readlines()
-        with open(nsfile, 'w') as f:
+        lines = []
         found = False
-            for line in lines:
+        with open(nsfile) as f:
+            for line in f.readlines():
                 if line.strip().startswith("Jupyter.version"):
-                    line = '    Jupyter.version = "{0}";\n'.format(version)
                     found = True
-                f.write(line)
+                    new_line = '    Jupyter.version = "{0}";\n'.format(version)
+                    if new_line == line:
+                        # no change, don't rewrite file
+                        return
+                    lines.append(new_line)
+                else:
+                    lines.append(line)
+
         if not found:
             raise RuntimeError("Didn't find Jupyter.version line in %s" % nsfile)
         
+        print("Writing version=%s to %s" % (version, nsfile))
+        with open(nsfile, 'w') as f:
+            for line in lines:
+                f.write(line)
+
 
 def css_js_prerelease(command, strict=False):
     """decorator for building minified js/css prior to another command"""