checkstyle: add SingleSpaceSeparator

Most of the code in our the code base only puts a single space between
non-whitespace characters,

    "Like " + "this."

It is highly likely that there will not be a case where we will need to
put more than a single space between non-whitespace characters,

    "Like "  + "this."

To enforce consistency throughout the code base, let's add the
SingleSpaceSeparator module to our checkstyle configuration, and fix all
of the existing violations in our code base.

build.gradle: upgrade TestFX to 4.0.7-alpha

TestFX 4.0.7-alpha has been released. Besides being the latest and
greatest version, 4.0.7-alpha also contains a proper working headless
mode for Linux.

Without this upgrade, attempting to run `./gradlew headless guiTests` on
Linux would result with the GUI tests failing with the following
exception:

        java.lang.RuntimeException: java.lang.RuntimeException: java.lang.IllegalAccessException: Class org.testfx.toolkit.impl.ApplicationLauncherImpl can not access a member of class com.sun.glass.ui.monocle.HeadlessPlatform with modifiers ""

            Caused by:
            java.lang.RuntimeException: java.lang.IllegalAccessException: Class org.testfx.toolkit.impl.ApplicationLauncherImpl can not access a member of class com.sun.glass.ui.monocle.HeadlessPlatform with modifiers ""

                Caused by:
                java.lang.IllegalAccessException: Class org.testfx.toolkit.impl.ApplicationLauncherImpl can not access a member of class com.sun.glass.ui.monocle.HeadlessPlatform with modifiers ""

Let's upgrade TestFX to 4.0.7-alpha to have happy Linux developers.

This working headless mode was also present in the 4.0.6-alpha, but we
couldn't upgrade to it as it broke our gui tests. (See e1eafd1 ([se-edu#353]
All gui tests are failing  (se-edu#354), 2017-03-23))

Furthermore, since headless mode now works on Linux, our Travis CI
builds do not need to run xvfb any more. As such, let's remove its usage
from .travis.yml.

[se-edu#621] UniquePersonList: Simplify logic of updatePerson(...) (s…

…e-edu#616)

This method retrieves the person in the list to be updated, and calls
Person#resetData(ReadOnlyPerson) to update the values of that person.

We do not need to retrieve the person in the list to be updated, to
update the list accordingly.

Let’s simplify the logic by setting the updated Person object in place
of the person to be updated.