NLL future-compat warning on nightly

[Nemo157]

Latest nightly has enabled the NLL migration mode for Edition 2015 code, this is triggering a couple of warnings (that will become errors in the future) in toml_edit 0.1.3:

warning[E0502]: cannot borrow `v` as immutable because it is also borrowed as mutable
  --> /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/toml_edit-0.1.3/src/index.rs:55:65
   |
44 |       fn index_or_insert<'v>(&self, v: &'v mut Item) -> &'v mut Item {
   |                          -- lifetime `'v` defined here
45 | /         match *v {
46 | |             Item::ArrayOfTables(ref mut vec) => {
47 | |                 vec.values.get_mut(*self).expect("index out of bounds")
48 | |             }
49 | |             Item::Value(ref mut a) if a.is_array() => a
   | |                         --------- mutable borrow occurs here
...  |
55 | |             _ => panic!("cannot access index {} in {:?}", self, v),
   | |                                                                 ^ immutable borrow occurs here
56 | |         }
   | |_________- returning this value requires that `v.0` is borrowed for `'v`
   |
   = warning: this error has been downgraded to a warning for backwards compatibility with previous releases
   = warning: this represents potential undefined behavior in your code and this warning will become a hard error in the future

warning[E0502]: cannot borrow `v` as immutable because it is also borrowed as mutable
  --> /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/toml_edit-0.1.3/src/index.rs:88:63
   |
70 |       fn index_or_insert<'v>(&self, v: &'v mut Item) -> &'v mut Item {
   |                          -- lifetime `'v` defined here
...
77 | /         match *v {
78 | |             Item::Table(ref mut t) => t.entry(self).or_insert(Item::None),
79 | |             Item::Value(ref mut v) if v.is_inline_table() => {
   | |                         --------- mutable borrow occurs here
80 | |                 &mut v
...  |
88 | |             _ => panic!("cannot access key {} in {:?}", self, v),
   | |                                                               ^ immutable borrow occurs here
89 | |         }
   | |_________- returning this value requires that `v.0` is borrowed for `'v`
   |
   = warning: this error has been downgraded to a warning for backwards compatibility with previous releases
   = warning: this represents potential undefined behavior in your code and this warning will become a hard error in the future