Fix line-directive print errors on different python versions

[hughbe]

Most Python files have

from __future__ import print_function

but this one doesn't, even though it has

print(map_line_from_source_file(source_file, source_line, target_file))

[hughbe]

@swift-ci please smoke test

[dabrahams]

This broke the doctests, that you get by running the tool with no arguments:

**********************************************************************
File "/Users/Shared/dabrahams/s/swift-master/utils/line-directive", line 231, in __main__.run
Failed example:
    print output.replace(target1.name,'TARGET1-NAME').replace(target2.name,'TARGET2-NAME') + 'EOF'
Exception raised:
    Traceback (most recent call last):
      File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/doctest.py", line 1315, in __run
        compileflags, 1) in test.globs
      File "<doctest __main__.run[13]>", line 1
        print output.replace(target1.name,'TARGET1-NAME').replace(target2.name,'TARGET2-NAME') + 'EOF'
                   ^
    SyntaxError: invalid syntax
**********************************************************************
File "/Users/Shared/dabrahams/s/swift-master/utils/line-directive", line 247, in __main__.run
Failed example:
    print subprocess.check_output([__file__, 'foo.bar', '21', target1.name]),
Exception raised:
    Traceback (most recent call last):
      File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/doctest.py", line 1315, in __run
        compileflags, 1) in test.globs
      File "<doctest __main__.run[14]>", line 1
        print subprocess.check_output([__file__, 'foo.bar', '21', target1.name]),
                       ^
    SyntaxError: invalid syntax
**********************************************************************
File "/Users/Shared/dabrahams/s/swift-master/utils/line-directive", line 249, in __main__.run
Failed example:
    print subprocess.check_output([__file__, 'foo.bar', '8', target2.name]),
Exception raised:
    Traceback (most recent call last):
      File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/doctest.py", line 1315, in __run
        compileflags, 1) in test.globs
      File "<doctest __main__.run[15]>", line 1
        print subprocess.check_output([__file__, 'foo.bar', '8', target2.name]),
                       ^
    SyntaxError: invalid syntax
**********************************************************************
1 items had failures:
   3 of  16 in __main__.run
***Test Failed*** 3 failures.