Make DBT dependency optional

[erezsh]

Fixes Issue #412

[erezsh]

@dlawin Any idea why these tests are failing?

[dlawin]

@dlawin Any idea why these tests are failing?

Hopefully it's clear from this: 21804e5

Need to use the mock_self as the mock, since that's how the methods are being called now. No need to mock the package (and a package level mock won't override the self.blah methods now)

[dlawin]

@dlawin Any idea why these tests are failing?

Hopefully it's clear from this: 21804e5

Need to use the mock_self as the mock, since that's how the methods are being called now. No need to mock the package (and a package level mock won't override the self.blah methods now)

I can fix the remaining ones later today

[erezsh]

@dlawin Nice, thank you!