Add support for Clojure 1.9

[axrs]

Should resolve #73

My test output reports no issues with the upgrade.

Thanks for the awesome work. It made a side project of combining multiple spread sheets together soo much easier.

---

lein test dk.ative.docjure.spreadsheet-test
#<XSSFCell temp>

lein test dk.ative.docjure.xls-test

Ran 70 tests containing 415 assertions.
0 failures, 0 errors.
Performing task 'test' with profile(s): '1.6'

lein test dk.ative.docjure.spreadsheet-test
#<XSSFCell temp>

lein test dk.ative.docjure.xls-test

Ran 70 tests containing 415 assertions.
0 failures, 0 errors.
Performing task 'test' with profile(s): '1.7'

lein test dk.ative.docjure.spreadsheet-test
#object[org.apache.poi.xssf.usermodel.XSSFCell 0x2726a511 "temp"]

lein test dk.ative.docjure.xls-test

Ran 70 tests containing 415 assertions.
0 failures, 0 errors.
Performing task 'test' with profile(s): '1.8'

lein test dk.ative.docjure.spreadsheet-test
#object[org.apache.poi.xssf.usermodel.XSSFCell 0x4a31ed12 "temp"]

lein test dk.ative.docjure.xls-test

Ran 70 tests containing 415 assertions.
0 failures, 0 errors.
Performing task 'test' with profile(s): '1.9'

lein test dk.ative.docjure.spreadsheet-test
#object[org.apache.poi.xssf.usermodel.XSSFCell 0x1796b2d4 "temp"]

lein test dk.ative.docjure.xls-test

Ran 70 tests containing 415 assertions.
0 failures, 0 errors.

[axrs]

@mjul Done. Thanks :)