deskutils/taskwarrior: Update to 2.5.0, Modernize

• Update PORTVERSION and distinfo checksum (2.5.0)
• Update pkg-plist
• Un-conditionalize GNUTLS. It is now mandatory [1]
• Switch to GITHUB for DISTFILES, tarball doesn't include tests
• Use USES=compiler for determing which C++11 compiler to use, deprecate OSVERSION < 10 check accordingly.
• Patch files to correct for ambiguous type signatures under LLVM, fixing build failure on 32-bit ARCH's (only i386 tested) and ISO8601 unit test failures. [3]
• Add python:test for TEST_DEPENDS support [2]
• Add do-test target for running the test suite
• Update SHEBANG_FILES, no more .py, .rb files
• Remove WRKSRC override
• Add LICENSE_FILE
• Sort Makefile sections

Changes:

[1] https://taskwarrior.org/news/news.20151021.html

[2] https://svnweb.freebsd.org/changeset/ports/405075
[3] C++ fixes kindly supplied by Brendan Molloy

Approved by: skreuzer (maintainer)
Differential Revision: D4756