Pin PyInstaller to version 3.4

Later versions cause problems.

setup.py

@@ -39,7 +39,7 @@ def _get_package_data(pkg_dir, data_subdir):
             'fbs/installer/mac', 'create-dmg'
         )
     },
-    install_requires=['PyInstaller>=3.3'],
+    install_requires=['PyInstaller==3.4'],
     extras_require={
         # Also update requirements.txt when you change this:
         'licensing': ['rsa>=3.4.2'],