See if we can get libusb to load in travis

.travis.yml

@@ -4,6 +4,11 @@ language: generic
 env:
     - SW=$HOME/sw PATH=$SW/bin:$PATH CFLAGS=-I$SW/include LDFLAGS=-L$SW/lib LD_LIBRARY_PATH=$SW/qt/lib:$SW/lib PKG_CONFIG_PATH=$SW/lib/pkgconfig QMAKE=$SW/qt/bin/qmake QT_PLUGIN_PATH=$SW/qt/plugins
 
+addons:
+    apt:
+        packages:
+            - libudev-dev
+
 before_install:
     - curl https://download.calibre-ebook.com/travis/sw-linux.tar.xz | tar xJ -C $HOME
     - python setup.py bootstrap --ephemeral

src/calibre/test_build.py

@@ -14,7 +14,6 @@ Test a binary calibre build to ensure that all needed binary images/libraries ha
 
 import os, ctypes, sys, unittest
 from calibre.constants import plugins, iswindows, islinux, isosx
-is_travis = os.environ.get('TRAVIS') == 'true'
 
 class BuildTest(unittest.TestCase):
 
@@ -60,8 +59,6 @@ class BuildTest(unittest.TestCase):
 
     def test_plugins(self):
         for name in plugins:
-            if is_travis and name in ('libusb', 'libmtp'):
-                continue
             mod, err = plugins[name]
             self.assertFalse(err or not mod, 'Failed to load plugin: ' + name + ' with error:\n' + err)
 
@@ -128,7 +125,6 @@ class BuildTest(unittest.TestCase):
         self.assertGreaterEqual(len(QFontDatabase().families()), 5, 'The QPA headless plugin is not able to locate enough system fonts via fontconfig')
         na = QNetworkAccessManager()
         self.assertTrue(hasattr(na, 'sslErrors'), 'Qt not compiled with openssl')
-        if not is_travis:
         from PyQt5.QtWebKitWidgets import QWebView
         QWebView()
         del QWebView