fix failure of module setup due to version.py not being available yet

https://travis-ci.org/fphammerle/freesurfer-volume-reader/jobs/528235118#L203

freesurfer_volume_reader/__init__.py

@@ -22,9 +22,11 @@ import typing
 
 import pandas
 
-import freesurfer_volume_reader.version
-
-__version__ = freesurfer_volume_reader.version.version
+try:
+    import freesurfer_volume_reader.version
+    __version__ = freesurfer_volume_reader.version.version
+except ModuleNotFoundError:  # pragma: no cover
+    __version__ = None
 
 
 def parse_version_string(version_string: str) -> typing.Tuple[typing.Union[int, str]]: