Force migrate 'alter' on tests

[dmarcelino]

Force default migrate: 'alter' so it's independent from waterline core's default: balderdashy/waterline#887

Addresses balderdashy/waterline#887

[devinivy]

Seems good to me ✔️

[dmarcelino]

@tjwebb, if we compare the latest master build (using the merged new default balderdashy/waterline#962) we can see:

 ------------------------------------------------------------------- 
| adapter          | version | status  | failed | total | wl-sequel |
|------------------|---------|---------|--------|-------|-----------|
| sails-postgresql | 0ad94e5 | failed  |      6 |     6 |     0.3.0 |
| sails-memory     | 7cf3600 | success |      0 |   207 |           |
| sails-disk       | 5210c18 | success |      0 |   209 |           |
| sails-mongo      | 414d748 | failed  |      5 |   210 |           |
| sails-mysql      | d289f11 | failed  |      2 |     2 |     0.3.0 |
| sails-redis      | 6eb8f92 | failed  |     29 |   196 |           |
 ------------------------------------------------------------------- 

On sails-postgresql and sails-mysql most tests don't even run because of migrate: 'safe' being the default.

On the other hand this PR's build brings the results to normality:

 ------------------------------------------------------------------- 
| adapter          | version | status  | failed | total | wl-sequel |
|------------------|---------|---------|--------|-------|-----------|
| sails-postgresql | 0ad94e5 | failed  |      2 |   228 |     0.3.0 |
| sails-memory     | 7cf3600 | success |      0 |   211 |           |
| sails-disk       | 5210c18 | success |      0 |   209 |           |
| sails-mongo      | 414d748 | failed  |      5 |   209 |           |
| sails-mysql      | d289f11 | failed  |      3 |   235 |     0.3.0 |
| sails-redis      | 6eb8f92 | failed  |     42 |   209 |           |
 ------------------------------------------------------------------- 

With only the known/expected errors. Any other concerns or shall we merge this as well?