context sourcing broken (ResolvedContext.append_sys_path not serialised)

[nerdvegas]

PR #878 added member ResolvedContext.append_sys_path, however serialization wasn't updated to reflect this. Consequently, any context loaded from file does not work:

$ rez-env -i test.rxt
Traceback (most recent call last):
...
  File "/home/ajohns/rez/2.61.0/local/lib/python2.7/site-packages/rez/resolved_context.py", line 1719, in _execute
    if self.append_sys_path:
AttributeError: 'ResolvedContext' object has no attribute 'append_sys_path'

A basic context serialisation error like this should have been caught by tests also, so part of this ticket should included adding the relevant unit test.