By FEMI SALAWU   Hennessy Artistry has unveiled its headliners for the year. They include the trio of Praiz, Burna Boy and Vector.   In its usual tradition, the artistes are expected to headline the Hennessy Artistry 2015 tour and produce a theme song.   According to Lere Awokoya, Hennessy Brand Manager, the premium cognac is celebrating its 250th anniversary …