Fix date column; update Exceljs version to 3.5.0

[nfreear]

Hi @vikramthyagarajan,

I've fixed the following in the pull request:

• Fixed date column data-type (Bug: Its not working Date Excel columns #6);
• Added missing package-lock.json;
• Fixed error in 'before all' hook in 'Excel writer' unit tests;
• Updated the Node versions in .travis.yml to 8.3 and 12 (Exceljs compatibility etc.);
• Updated Exceljs from 0.2.x to 3.5.0 (Pull requests: update exceljs version #4, changed min version of mocha, exceljs #5, Bump exceljs from 0.2.46 to 3.3.1 #8);
• Updated Mocha from 3.x to 7.x;
• Fixed "11 vulnerabilities (4 low, 1 moderate, 5 high, 1 critical)" ~ via: npm audit
• Bumped the package version to 1.2.0;

I hope this helps!

Nick

[vikramthyagarajan]

Thank you for this @nfreear :)