bump node requirement 0.7.0 for libuv's uv_mutex_t

uv_mutex_t was added to node via libuv in node 0.7.0.
Builds fail otherwise.  This doesn't seem to help my case, but 1.0.6, before the synching stuff seems ok.

    bewest@ripen:~/Documents/git/node$ node -v
    v0.6.17
    bewest@ripen:~/Documents/git/node$ git log --graph -Suv_mutex_t working-2012-09-09...v0.6.21 
    bewest@ripen:~/Documents/git/node$ 
    bewest@ripen:~/Documents/git/node$ git log --graph -Suv_mutex_t working-2012-09-09...v0.7.0  | grep commit | wc
         10      39     518



# npm install
    bewest@ripen:~/Documents/git/node-serialport$ npm install 

    > serialport@1.0.7 install /home/bewest/Documents/git/node-serialport
    > node-gyp rebuild

    info it worked if it ends with ok 
    spawn python [ '/home/bewest/.node-gyp/0.6.17/tools/gyp_addon',
      'binding.gyp',
      '-I/home/bewest/Documents/git/node-serialport/build/config.gypi',
      '-f',
      'make' ]
    spawn make [ 'BUILDTYPE=Release', '-C', 'build' ]
    make: Entering directory `/home/bewest/Documents/git/node-serialport/build'
      CXX(target) Release/obj.target/serialport/src/serialport.o
    ../src/serialport.cpp:8: error: ‘uv_mutex_t’ does not name a type
    ../src/serialport.cpp: In function ‘v8::Handle<v8::Value> Open(const v8::Arguments&)’:
    ../src/serialport.cpp:14: error: ‘write_queue_mutex’ was not declared in this scope
    ../src/serialport.cpp:14: error: ‘uv_mutex_init’ was not declared in this scope
    ../src/serialport.cpp: In function ‘v8::Handle<v8::Value> Write(const v8::Arguments&)’:
    ../src/serialport.cpp:112: error: ‘write_queue_mutex’ was not declared in this scope
    ../src/serialport.cpp:112: error: ‘uv_mutex_lock’ was not declared in this scope
    ../src/serialport.cpp:120: error: ‘uv_mutex_unlock’ was not declared in this scope
    ../src/serialport.cpp: In function ‘void EIO_AfterWrite(uv_work_t*)’:
    ../src/serialport.cpp:139: error: ‘write_queue_mutex’ was not declared in this scope
    ../src/serialport.cpp:139: error: ‘uv_mutex_lock’ was not declared in this scope
    ../src/serialport.cpp:148: error: ‘uv_mutex_unlock’ was not declared in this scope
    make: *** [Release/obj.target/serialport/src/serialport.o] Error 1
    make: Leaving directory `/home/bewest/Documents/git/node-serialport/build'
    ERR! Error: `make` failed with exit code: 2
        at ChildProcess.onExit (/usr/local/lib/node_modules/npm/node_modules/node-gyp/lib/build.js:209:23)
        at ChildProcess.emit (events.js:70:17)
        at maybeExit (child_process.js:362:16)
        at Process.onexit (child_process.js:398:5)
    ERR! not ok

    npm ERR! serialport@1.0.7 install: `node-gyp rebuild`
    npm ERR! `sh "-c" "node-gyp rebuild"` failed with 1
    npm ERR! 
    npm ERR! Failed at the serialport@1.0.7 install script.
    npm ERR! This is most likely a problem with the serialport package,
    npm ERR! not with npm itself.
    npm ERR! Tell the author that this fails on your system:
    npm ERR!     node-gyp rebuild
    npm ERR! You can get their info via:
    npm ERR!     npm owner ls serialport
    npm ERR! There is likely additional logging output above.
    npm ERR! 
    npm ERR! System Linux 2.6.32-42-generic
    npm ERR! command "node" "/usr/local/bin/npm" "install"
    npm ERR! cwd /home/bewest/Documents/git/node-serialport
    npm ERR! node -v v0.6.17
    npm ERR! npm -v 1.1.21
    npm ERR! code ELIFECYCLE
    npm ERR! message serialport@1.0.7 install: `node-gyp rebuild`
    npm ERR! message `sh "-c" "node-gyp rebuild"` failed with 1
    npm ERR! errno {}
    npm ERR! 
    npm ERR! Additional logging details can be found in:
    npm ERR!     /home/bewest/Documents/git/node-serialport/npm-debug.log
    npm not ok
    bewest@ripen:~/Documents/git/node-serialport$