Another attempt to fix httpbin in Travis CI Trusty build

Somehow installing decorator==4.4.2 worked a few times, but then stopped
and now fails with

	Could not find a version that satisfies the requirement
	decorator==4.4.2 (from versions: )

	No matching distribution found for decorator==4.4.2

Try using explicit URL.

build/tools/travis-ci.sh

@@ -26,13 +26,21 @@ launch_httpbin() {
     # version there is too old to support SNI.
     case "$dist_codename" in
         trusty)
-            pip_args='--trusted-host files.pythonhosted.org'
+            # Here "decorator==4.4.2" doesn't work neither for some reason ("no
+            # matching distribution found"), so use an explicit URL.
+            pip_decorator_arg='https://files.pythonhosted.org/packages/ed/1b/72a1821152d07cf1d8b6fce298aeb06a7eb90f4d6d41acec9861e7cc6df0/decorator-4.4.2-py2.py3-none-any.whl'
+            pip_options='--trusted-host files.pythonhosted.org'
+            ;;
+
+        *)
+            # Current decorator (>= 5) is incompatible with Python 2 which we
+            # still use under in some builds, so explicitly use a version which
+            # is known to work.
+            pip_decorator_arg='decorator==4.4.2'
             ;;
     esac
 
-    # decorator 5+ is incompatible with Python 2 which we still use under in
-    # some builds, so explicitly select version 4.4 which is known to work.
-    pip install decorator==4.4.2 httpbin --user $pip_args
+    pip install $pip_decorator_arg httpbin --user $pip_options
     python -m httpbin.core &
     WX_TEST_WEBREQUEST_URL="http://localhost:5000"