Tests working with rlang 0.3.0 dev version

[clarabicalho]

Closes #248

[jaspercooper]

This looks good to me, locally and seems OK on Travis

[jaspercooper]

@nfultz FYI this is the error message the rlang team sent us, which indeed seems generated by the lines @clarabicalho fixed:

== CHECK RESULTS ========================================

• checking tests ...

   ERROR
  Running the tests in ‘tests/testthat.R’ failed.
  Last 13 lines of output:
               }
           }
       }, args = c("x"), arguments_as_values = F, exclude_args =
  NULL), "character") at testthat/test_helpers.R:37
    2: quasi_label(enquo(object), label, arg = "object")
    3: force(quo)
    4: enquo(object)
    5: rlang::abort(x) at
  /private/var/folders/b9/1vbq6rn93_1fk71sn95dqb8r0000gn/T/Rtmp3Ij4Yr/R.INSTALL917422a6e5c/rlang/R/quotation.R:293
  
    ══ testthat results
  ═══════════════════════════════════════════════════════════
    OK: 309 SKIPPED: 0 WARNINGS: 5 FAILED: 2
    1. Error: internal helpers for when source code is missing work
  (@test_helpers.R#31)
    2. Error: construct_design_code works as it should when source
  is missing (@test_helpers.R#37)
  
    Error: testthat unit tests failed
    Execution halted