Convert dict to str explicitly

When there is an error in a `.bib` file the `latex_cite_completions.py` file prints a message to the user, and in the process attempts to print the previous entry, which is a dictionary object. For me that results in an error:
```
Cannot process this @ line: @INPROCEEDINGS{,
Traceback (most recent call last):
  File "/Applications/Sublime Text.app/Contents/MacOS/sublime_plugin.py", line 549, in run_
    return self.run(edit)
  File "/Users/bartvanmerrienboer/Library/Application Support/Sublime Text 3/Packages/LaTeXTools/latex_cite_completions.py", line 417, in run
    completions, prefix, post_brace, new_point_a, new_point_b = get_cite_completions(view, point)
  File "/Users/bartvanmerrienboer/Library/Application Support/Sublime Text 3/Packages/LaTeXTools/latex_cite_completions.py", line 287, in get_cite_completions
    print ("Previous record " + entry)
TypeError: Can't convert 'dict' object to str implicitly
```
As a result the `\cite` auto-completion doesn't work anymore. Using the `str` method seems to solve the problem for me.