Revert to xterm <= 4.2.0 to avoid theme/style related issues

We can't upgrade xterm yet because of xtermjs/xterm.js#2620.
Our theme values are inherited from our CSS variables, which
can include rgba() values, and currently only #rrggbbaa is
supported.

packages/terminal/package.json

@@ -41,7 +41,7 @@
     "@lumino/domutils": "^1.1.5",
     "@lumino/messaging": "^1.3.1",
     "@lumino/widgets": "^1.9.4",
-    "xterm": "~4.3.0",
+    "xterm": "~4.2.0",
     "xterm-addon-fit": "~0.3.0"
   },
   "devDependencies": {