fix(tests): use cultofcoders:mocha to support Meteor > 1.6.1

[lmachens]

Issue:
meteor test-packages ./ --driver-package practicalmeteor:mocha is failing for Meteor > 1.6.1 (see practicalmeteor/meteor-mocha#100).
Error:

I20180515-12:12:19.135(2)? Exception while invoking method 'mocha/runServerTests' Error: invalid reporter "[object Object]"
I20180515-12:12:19.136(2)?     at Mocha.reporter (C:\Users\lmachens\AppData\Local\.meteor\packages\practicalmeteor_mocha-core\1.0.1\npm\node_modules\mocha\lib\mocha.js:180:13)
I20180515-12:12:19.137(2)?     at MochaRunner.coffee.js.MochaRunner.runServerTests (packages/practicalmeteor_mocha/meteor/src/lib/MochaRunner.coffee:72:19)
I20180515-12:12:19.137(2)?     at MochaRunner.runServerTests (packages/practicalmeteor_mocha/meteor/src/lib/MochaRunner.coffee:1:1)
I20180515-12:12:19.139(2)?     at maybeAuditArgumentChecks (packages/ddp-server/livedata_server.js:1768:12)
I20180515-12:12:19.139(2)?     at DDP._CurrentMethodInvocation.withValue (packages/ddp-server/livedata_server.js:719:19)
I20180515-12:12:19.140(2)?     at Meteor.EnvironmentVariable.EVp.withValue (packages\meteor.js:1186:12)
I20180515-12:12:19.140(2)?     at DDPServer._CurrentWriteFence.withValue (packages/ddp-server/livedata_server.js:717:46)
I20180515-12:12:19.141(2)?     at Meteor.EnvironmentVariable.EVp.withValue (packages\meteor.js:1186:12)
I20180515-12:12:19.142(2)?     at Promise (packages/ddp-server/livedata_server.js:715:46)
I20180515-12:12:19.142(2)?     at new Promise (<anonymous>)
I20180515-12:12:19.143(2)?     at Session.method (packages/ddp-server/livedata_server.js:689:23)
I20180515-12:12:19.143(2)?     at packages/ddp-server/livedata_server.js:559:43

Solution:
cultofcoders:mocha works fine.

Other:
Tests are failing (should be fixed in other PR):