curl (or sbt) should be included in the nix default environment

[smurching]

When trying to build tensorframes (e.g. ./build/sbt distribution/compile) within the default nix shell, sbt tries to download itself using curl, which fails as curl is not installed by default.

Adding curl as a dependency in default.nix may fix this issue.

[smurching]

For now, a workaround is to install curl in the nix environment via the following steps:

1. Add apt to your path by running $ export PATH=$PATH:/usr/bin
2. Install curl via $ sudo apt install curl