Allow any string for device_region

- Existing clients/devices are reporting
  values such as mobile provider name.
  The existing validation prevents these
  clients from connecting.

psiphon/server/api.go

@@ -531,7 +531,7 @@ var baseRequestParams = []requestParamSpec{
 	{"client_build_rev", isHexDigits, requestParamOptional},
 	{"relay_protocol", isRelayProtocol, 0},
 	{"tunnel_whole_device", isBooleanFlag, requestParamOptional},
-	{"device_region", isRegionCode, requestParamOptional},
+	{"device_region", isAnyString, requestParamOptional},
 	{"ssh_client_version", isAnyString, requestParamOptional},
 	{"upstream_proxy_type", isUpstreamProxyType, requestParamOptional},
 	{"upstream_proxy_custom_header_names", isAnyString, requestParamOptional | requestParamArray},