Just add this to your ~/.m2/settings.xml:
<profiles>
<profile>
<id>allow-snapshots</id>
<activation><activeByDefault>true</activeByDefault></activation>
<repositories>
<repository>
<id>snapshots-repo</id>
<url>https://oss.sonatype.org/content/repositories/snapshots</url>
<releases><enabled>false</enabled></releases>
<snapshots><enabled>true</enabled></snapshots>
</repository>
</repositories>
</profile>
</profiles>